It's generally not like a store, it's charged at time of import not at time of sale. The seller can choose to pre-pay for the importer (the customer) but they'll only do that if they know what to pay. If they don't know what to pay (i.e. if the tariffs change randomly with no notice) they'll tick the box that says importer pays and the buyer gets a call from customs when it arrives in the country saying pay the tariff or we impound/destroy your goods.